1. Field of the Invention
This invention relates to an alignment apparatus whereby two workpieces disposed optically opposedly may be aligned into a predetermined positional relationship.
2. Description of the Prior Art
Alignment of two workpieces has often been practised in aligning a mask and a wafer for producing a semiconductor circuit element. Therefore, the following description will be made by taking the alignment of a mask and a wafer as as example.
Usually, a mask is supported by a fixed mask carrier and a wafer is supported by a wafer carrier which is parallel-movable in the directions x, y and .theta. with respect to the mask carrier. The mask and wafer are provided with respective alignment marks and by photoelectrically scanning these alignment marks along a predetermined scanning line, the positional relation between the standard point on the mask and a point on the wafer corresponding to the standard point on the mask is measured and, when the point on the wafer is without the allowable error range relative to the standard point on the mask, the wafer carrier is moved in a plane by driving x, y and .theta. motors, thereby effecting the alignment within the aforementioned allowable error range.
Alignment marks are usually provided at two locations on the mask and wafer along a predetermined standard line. The two locations are usually situated at an interval so as to be symmetrical with respect to the centers of the mask and wafer.
Whether or not the alignment between the standard point on the mask and the corresponding point on the wafer is within an allowable error range is discriminated by a circuit for discriminating a relative position signal provided by a photoelectric output. This discrimination circuit determines whether or not the corresponding point on the wafer lies within a square, circular or diamond-shaped region having the standard point on the mask as the center.
However, the interval between the two alignment marks on the mask and wafer is varied by a manufacturing error or the like. Such variation in interval is called the pitch error. When such pitch error is present, it is conceivable to enlarge the area of the region by an amount corresponding to the pitch error, but this is not desirable in that it greatly alters the allowable area.